This invention relates to a multilayer flexible transfer tape comprising a backing and a layer of pressure-sensitive adhesive, at least one binder-containing pigmented transfer layer being present between the backing and the layer of pressure-sensitive adhesive and showing greater adhesion to the layer of pressure-sensitive adhesive than to the backing.
A transfer tape of the type described above is known, for example, from DE 196 17 850 C1. This document describes a transfer tape in which a pigmented, particularly white-pigmented, transfer layer, then another pigmented transfer layer containing a non-white pigment and finally the layer of contact adhesive are arranged on a conventional backing. This tape is particularly effective in covering the transfer layer without any significant loss of xe2x80x9cwhitenessxe2x80x9d. The known tape is advantageously used in roll form in hand-held xe2x80x9crollersxe2x80x9d so that the transfer layer can be applied simply, quickly and uniformly to a substrate in order to cover printing/typing errors in texts or drawings so that corrections may be made. The transfer layer applied can then be written on, for example with a fountain pen or ball-point pen. In some cases, it has been found in connection with such corrections that the dyes in the lettering covered by the tape migrate to the surface of the covering layer applied, particularly under the influence of moisture, so that the covered letting can be seen again.
Accordingly, the problem addressed by the present invention was to further develop the transfer tape mentioned at the beginning in such a way that the xe2x80x9cstrike-throughxe2x80x9d of covered lettering would be eliminated without any adverse effect on the desirable properties, particularly the covering power of the transfer layer.
According to the invention, the solution to this problem is characterized in that the transfer tape comprises two binder-containing pigmented transfer layers, one of the pigmented transfer layers being cationic and the other pigmented transfer layer being anionic.
Advantageous embodiments of the present invention are defined in the subsidiary claims. In these embodiments, the two transfer layers have a thickness of about 5 to 25 g/m2 and a total thickness of about 15 to 30 g/m2. The separate adhesive layer preferably has a thickness of about 1 to 5 g/m2 and more preferably in the range from about 2 to 4 g/m2. Particularly good effects are obtained if a fine-particle metal powder, more particularly fine-particle aluminium, is present in the anionic transfer layer and/or in the adhesive layer. The fine-particle metal powder is preferably present in the form of platelets. The platelet-like aluminium particles advantageously have a thickness of about 3 to 10 xcexcm and a diameter of about 4 to 17 xcexcm. The quantity of fine-particle metal powder used is between about 0.1 and 3.5% by weight, based on the particular dry layer. Particularly favorable effects are obtained if the anionic transfer layer is located between the cationic transfer layer and the adhesive layer. If the fine-particle metal powder is present in the adhesive layer, the sequence of the anionic and cationic transfer layers is of no relevance. In individual cases, it is of particular advantage if, as seen from the backing, the cationic transfer layer is applied first, followed by an adhesive, anionic transfer layer with no further layer of pressure-sensitive adhesive, the adhesive anionic transfer layer in particular containing fine-particle metal, more particularly fine-particle aluminium.
The basic concept of the present invention is that, irrespective of the sequence in the layer structure of the transfer tape, the transfer tape comprises a cationic and additionally an anionic transfer layer. The terms xe2x80x9canionicxe2x80x9d and xe2x80x9ccationicxe2x80x9d will readily be understood by the expert. The transfer layers are preferably prepared using binder dispersions, i.e. dispersions containing solid small polymer particles, more particularly in aqueous form. Commercially obtainable cationic and anionic aqueous dispersions may be used, for example cationic aqueous dispersions commercially obtainable as Acronal(copyright)) 280 KD (from BASF AG), Butonal(copyright) LS 170 K (from BASF AG), Jagotex(copyright) AL 2463 (from Jxc3xa4ger), cationic solutions obtainable, for example, as Worleecryl(copyright) (cationic pure acrylate solution, clear to slightly milky solution, on the one hand 25% in water (7712 W) and on the other hand 40% in water/isopropanol (40:20) (7712), pH value 5 in either case) (from Worlee Chemie GmbH, Hamburg) and anionic aqueous dispersions obtainable as Acronal(copyright) S 725 and S 726 (butyl acrylate/styrene copolymer) (from BASF AG), as Acronal(copyright) V 205 (from BASF AG), as Styrofan(copyright) D 422 and Propiofan(copyright) 6D (from BASF AG). The activities mentioned are attributable to the particular surfactant used in the emulsion polymerization process. In the case of an anionic dispersion, anionic surfactants a carboxyl group (xe2x80x94COOxe2x88x92) are used as stabilizers in the emulsion polymerization. These groups face outwards from the dispersed polymer particles. In the case of the cationic dispersions, cationic surfactants which almost without exception contain a quaternary ammonium ion (xe2x80x94N+(R3)) as hydrophilic group are used in the emulsion polymerization process. This information is all the expert needs to choose suitable commercially available ionic aqueous dispersions for achieving the objects of the invention. Reference is made in this connection to Dr. H. Stache xe2x80x9cTensid-Taschenbuchxe2x80x9d, Carl Hanser Verlag Mxc3xcnchen/Wien, 1979, pp. 2/3 and Rxc3x6mpps Chemie-Lexikon, 7th Edition, Georg Thieme Verlag, 1992, Vol. 6, p. 4495, right-hand column to 4499, left-hand column.
Accordingly, the above-mentioned aqueous dispersions and solutions are preferably used in the production of the transfer layers to be formed in accordance with the invention. The above list of aqueous dispersions/solutions is by no means complete and, in particular, is not limiting in regard to their choice. On the contrary, it is quite clear to the expert that other binders may also be used, especially since the essence of the invention does not lie in the type of binder used, but solely in the cationic or anionic character of the transfer layer. The concentration of the binder in the dispersion is not critical. As a rough guide, it may be between about 25 and 70% by weight and is preferably between about 40 and 60% by weight. The aqueous dispersion for forming the transfer layers is applied to the backing in a quantity of preferably about 15 to 35 g/m2 (dry weight) and more preferably about 18 to 25 g/m2, this quantity representing the total quantity of both transfer layers, i.e. the cationic and anonic transfer layer.
Typical additives, for example foam inhibitors, wetting agents and the like, may be used in the production of the various layers.
The layer of pressure-sensitive adhesive may consist of commercially available pressure-sensitive adhesives, for example the Freihoff-Dispersion VP 859/6. The above-described materials of the individual layers of the transfer tape according to the invention generally satisfy the basic requirement that the adhesive tension between the backing and the transfer layers mentioned is lower than between the transfer layers and the layer of pressure-sensitive adhesive (cf. DE 196 17 850 C1).
Technologically, the invention may be explained as follows: most of the dyes in writing fluids are synthetic dyes based on aromatic or heterocyclic compounds. The dyes are either ionic (for example all water-soluble dyes) and nonionic compounds (for example dispersions dyes). Among the ionic dyes, there are anionic and cationic types. The anionic dyes have a negatively charged dye ion while the cationic dyes have a positively charged dye ion so that, previously, a distinction was generally drawn between acidic and basic dyes. It has been found that, irrespective of the particular type of lettering, i.e. whether it contains an anionic or cationic dye, the present inventionxe2x80x94in the covering of a writing/printing error for examplexe2x80x94enables the troublesome migration of the dyes through the covering layer to be eliminated. If the lettering contains an anionic dye, the cationic transfer layer of the transfer tape according to the invention blocks any migration. In the case of a cationic dye, this blocking effect is developed by the anionic covering layer (cationic dye binds to the anionic layer, etc.). Here, the dye does not migrate through the polymer itself, but through microvoids present in the transfer layer. If the dye migrates through those voids, it automatically comes into contact with the active and outwardly directed parts of the surfactants present on the surface of the polymer particles and is arrested by corresponding interaction.
The advantages obtainable through the invention may be summarized as follows: the above-mentioned strike-through of covered lettering is eliminated in the required manner without any adverse effect on the desirable properties or on the covering power of the transfer layer. Writing/printing can be permanently covered.